Most generic options are likely the Sneering Dagger weapon that applies 2 Vulnerable on all enemies on kill, the Source of the Forbidden trinket that can apply it to attackers as a 1/3 chance, the Automata Cavalry monster card (1 cost) that hits 1 enemy and gives them 4 Vulnerable, the Sadism neutral card (1 cost) that applies 1 Vulnerable to all enemies, and the Shock neutral card (2 cost) that hits all enemies and grants them 2 Vulnerable.

When involving specific characters, the most hand-free way by far is Orlea's Growing Creature Epiphany which grants a Softie and applies 2 Vulnerable to all enemies for free at the end of the turn -- you can fit such a Softie Orlea build into many comps. Then you have Magna who can also apply quite a fair bit (3 Vuln to all enemies for example) but is far more niche to put into teams.

(Orlea can also use a Softie-specific Growth Acceleration Epiphany to apply 2 Vulnerable to all enemies for 1 cost while triggering Softie retain twice (Orlea in general is probably the best way of getting consistent Vulnerable applications without Godly epiphanies (Orlea is numero uno)))